1. Field of the Invention
The present invention relates to an amplification circuit for amplifying a control signal reproduced by a control head of a video tape reproducing apparatus and particularly to a control signal amplification circuit in which the quantity of negative feedback is controlled so that the output level of the amplification circuit is kept constant.
2. Description of the Related Art
In a video tape reproducing apparatus such as a video cassette deck, or the like, the rotation of a rotary head and the running of a video tape are synchronized with each other on the basis of the output of a control head for reproducing a control signal when the video tape is made to run. On the other hand, a magnetic head wound with a coil is used as the control head. Accordingly, when the running speed of the video tape varies in accordance with the operation difference in kind, for example, between "standard-playing mode" and "long-playing mode" or between "normal play" and "special play", the output level of the control head changes greatly. Therefore, the amplification factor of an amplification circuit for amplifying the control signal reproduced by the control head must be changed in accordance with the running speed of the video tape.
FIG. 5 shows a conventional basic circuit in which the amplification factor can be changed. That is, in this conventional technique, the respective connection conditions of a plurality of switches 91, 92, . . . are changed to change the quantity of negative feedback of an operational amplifier 95 to thereby obtain the optimum amplification factor for the running speed of the video tape. In the aforementioned configuration, however, the respective connection conditions of the switches 91, 92, . . . must be controlled in accordance with the running speed of the video tape. Accordingly, there arises a problem that the control of the switches 91, 92, . . . becomes complex and therefore the program used in a microcomputer for controlling the switches becomes complex.
To solve this problem, it will do to provide an automatic level control (hereinafter merely referred to as "ALC") for automatically changing the amplification factor of the amplification circuit to thereby keep the output level of the operational amplifier constant. To this end, as shown in FIG. 6, a conventional amplification circuit having an ALC function has been proposed. That is, in the proposed technique, the control signal is led to an amplifier 97 through an attenuator 96 in which the quantity of attenuation changes in accordance with a control voltage. Further, the quantity of attenuation in the attenuator 96 is controlled in accordance with a control signal based on the output level of the amplifier 97 so that the output level of the amplifier 97 is kept constant.
In the case where the aforementioned configuration is used, however, a problem arises as follows. That is, since the through-rate of the operational amplifier used as the amplifier 97 is constant, the open-loop gain decreases as the frequency increases. On the other hand, in the configuration shown in FIG. 6, the operational amplifier constituting the amplifier 97 is set to have the optimum amplification factor when the level of the control signal is smallest. That is, the operational amplifier is set so as to operate with a very large amplification factor. As a result, the frequency range in which signal amplification can be performed with the set amplification factor is limited to a frequency range in which the open-loop gain of the operational amplifier is kept to have a sufficient value. Accordingly, to amplify the control signal without deformation when the video tape runs at a high speed, an expensive operational amplifier good in frequency characteristic must be used as the operational amplifier constituting the amplifier 97.